1. Field of the Invention
The present invention relates to a stator structure including coil covers, and relates, in particular, to a stator structure suitable for a resolver, and to a resolver.
2. Description of the Related Art
Resolvers are known as rotation angle sensors. For example, a resolver is formed of a stator core including teeth extending inwards to the center from an annular yoke portion and a rotor arranged opposite to the teeth portion of a stator inside the stator core. A winding wire is wound around the teeth via an insulator to form a coil. The winding wire is formed of an excitation winding wire for inputting excitation signals and two detecting winding wires for outputting two-phase signals according to rotation angle of the rotor. From the detecting winding wires, sine signals in the form of sin θ and cosine signals in the form of cos θ are output. A structure of a resolver is known, in which the structure has two coil covers attached to both sides of a stator core, the two coil covers covering a winding wire (coil) (see, for example, Japanese Patent Application Laid-open No. 2009-148081).
In the structure disclosed in Japanese Patent Application Laid-open No. 2009-148081, as illustrated in FIG. 7 and the like, a plurality of first support pillar portions and a plurality of first receiving portions are provided in the first coil cover, and a plurality of second support pillar portions and a plurality of second receiving portions are provided in the second coil cover. The first and second support pillar portions extend in spaces (slots) between teeth provided with an insulator. After the first and second coil covers are attached to the stator core, ultrasonic vibration is applied to positions where the second receiving portions of the second coil cover are provided, such that the first support pillar portions of the first coil cover and the second receiving portions of the second coil cover are welded to each other. Consequently, the first and second coil covers are joined to each other.
As a structure for joining two coil covers to each other, a structure has been known in which the structure uses a snap-fit between a locking projection and a locking stepped portion (see, for example, Japanese Patent Application Laid-open No. 2004-135402).
In a stator structure for a resolver, the stator structure disclosed in Japanese Patent Application Laid-open No. 2004-135402, the locking projection provided in the first coil cover penetrates a slot between teeth provided with an insulator, and engages with a cylindrical projection provided in the second coil cover, thereby joining the two coil covers together. After a locking claw of the locking projection penetrates by engaging with an annular tapered portion of the cylindrical projection, with the locking claw being deformed or distorted due to an expanding slot thereof, the locking claw is locked by the snap-fit with the locking stepped portion on a reverse side of the cylindrical projection.
In order to realize higher reliability in a stator of a resolver, arrival of foreign matter at a coil from an inner peripheral side of the stator needs to be prevented or reduced. Providing a protective structure covering an opening of a slot to the inner peripheral side of the stator is one method of realizing that. Such a protective structure may be realized, for example, by the support pillar portions and receiving portions, or the projections locking each other, which are described in the above cited Japanese patent applications.
However, since the support pillar portions and receiving portions, or the projections, of the above cited Japanese patent applications are also used as the structure for joining the two coil covers, upon the attachment of the coil covers, their mutual positional relation is fixed by the welding or locking. Therefore, if the welding or the locking is carried out when an error originated from the design is caused in the positions or sizes of the support pillar portions and receiving portions, or in the projections due to manufacture variation of the coil covers or the like; this error may result in distortion of the shape of the protective structure. The greater the number of the support pillar portions and receiving portions or of the projections, the greater the possibility and extent of such distortion will be. In this case, unintended gaps may be generated due to the distortion of the protective structure and foreign matter may enter the slots from the gaps. By this, reliability of the resolver may be deteriorated.